EXTRA RAM NOT WORKING ,HELP ME

I have an ASROCK K7S8x, AMD 2800+ with 512 DDR400(2.5)ram and have just bought another 512 DDR400(3) ram but when i plug it in the PC says it still only has 512 ram.I have tried taking the old 1 out and replacing it with the new 1 on its own and it works fine.This means both are being recognised seperately but not together.I don't know if this means anything,but on the side of the new ram their is a number 3 in brackets after it says DDR400 and on my original it has 2.5 in brackets after DDR400.I would appreciate any advice.

Hey! The 2.5 and 3 markings denote cas(column address strobe)latency times.This specifies the time in clock cycles from when the request is made by memory to the time it is received. The lower the number the faster the access time.Your board and bios are compatible with both speeds but not at the same time. One is working faster than the other and that spells trouble for some boards with ddr--I guess you have one!!
Replacing the new(slower)stick with a 2.5 is the best soloution providing your board supports 1G of Ram.
The key to a fast and stable system is identical ram!!!